The Granma was interesting to research. It's length was reported variously between sixty and eighty feet. The confusion may result from the fact that vessels may be measured along the keel or the deck or even the waterline. All three would be different for the same boat. Was it diesel powered or driven by gasoline engines? I found claims for both. However, all sources seem to agree that the Granma's engines were tired from years of use and that the vessel was seriously overburdened on the voyage to Cuba.
Inasmuch as I have the advantage of some experience as a sailor, I was able to calculate just how severely the Granma was tested. Approximately 82 men voyaged on the Granma from the Tuxpam River on the East Coast of Mexico to the eastern end of the island of Cuba. (I say approximately because reports vary.) That's about 70 more than I would have attempted to carry on that trip. Allowing 250 pounds for each man and the weapon, ammunition, and supplies he carried, I calculated that the Granma sailed with its normal load line (waterline) submerged by at least six inches. That may not seem alarming to you. The vessel would still have plenty of reserve buoyancy to remain afloat in a mill pond. Unfortunately, they weren't crossing a mill pond. Even more problematic, that load would have greatly raised the boat's center of gravity, making it extremely unstable.
Granma's buoyancy and stability were both put to the test as soon as it crossed the bar from the Tuxpam River into the Gulf of Mexico. Records show that the Fidelistas departed at approximately 1:00 am on November 25, 1956, to sneak past Mexican authorities in the dark. There was no guarantee that the Mexicans would permit their voyage to begin from their shores. A waning quarter moon would have provided the helmsman with just enough light to steer the channel, and not enough light to illuminate the yacht for sentries on shore.
In my novel, Rebels on the Mountain, I populated the Fidelistas with a young engineering student from Havana University, to provide readers with a character to interpret the scenario. I imagined that such a man, with his engineering knowledge, would have appreciated the dangers they were facing. I attempted to illustrate this fact for readers by showing his reluctance to board the yacht when he saw how heavily it was laden. I then demonstrated his commitment to the revolution by having him step aboard at the last second as they departed the wharf.